Magnetic Susceptibility and Reflectance Spectroscopy for Non-Invasive Detection of Cave Art Beneath Calcite

Researchers from Université Bourgogne Europe published research in the Journal of Cultural Heritage that shows how combining magnetic susceptibility and reflectance spectroscopy offers a promising, non-invasive way to detect prehistoric cave paintings hidden beneath protective calcite layers (1). Calcite is a carbonate mineral often found in caves (2).

In their study, the researchers used these two complementary and non-invasive techniques to study calcite layers and detect the prehistoric cave paintings underneath these layers. One of the biggest challenges in archaeology is uncovering cave paintings and petroglyphs that remain hidden under calcite deposits. Although these calcite deposits protect cave paintings from degradation, the deposits also make it harder for archaeologists to find and study cave paintings. As a result, traditional inventory and analysis is almost impossible (1). In addition, another challenge is that the mechanical removal of calcite is invasive and risky, and common imaging methods often fail to penetrate thick mineral layers effectively (1).

Alghero, Sardinia: Nettuno caves. Grotte di nettuno. | Image Credit: © nextyle - stock.adobe.com

What did the researchers do in their study?

As part of the experimental procedure, the research team explored how magnetic susceptibility and reflectance spectroscopy can be used to overcome the abovementioned challenges. Both techniques offered key advantages in the study. Magnetic susceptibility was used to measure the magnetic properties of materials, and it is sensitive to iron-based pigments such as hematite and goethite, which were commonly used in prehistoric art (1). Meanwhile, reflectance spectroscopy, which spans wavelengths from 400 to 2500 nanometers, provides detailed information about pigment composition and mineralogical contrasts (1).

The researchers tested their method by painting a limestone slab with various pigments and covering it with calcite layers of different thicknesses. Then, they applied both reflectance spectroscopy and magnetic susceptibility to analyze the pigments.

What were the results of the study?

The researchers achieved several encouraging results. They found that magnetic susceptibility could detect iron-based pigments beneath calcite layers up to 7 millimeters thick (1). Meanwhile, reflectance spectroscopy successfully distinguished spectral differences between all materials, though the strength of the signal decreased with increasing calcite thickness (1).

The researchers, apart from testing their method in a controlled laboratory environment, also applied it at the Grande Grotte of Arcy-sur-Cure in France, which is renowned for its prehistoric paintings. Three red ochre paintings concealed by calcite were analyzed. Magnetic susceptibility proved highly effective for detecting iron-rich pigments, while reflectance spectroscopy offered broader mineralogical insights (1). Together, the two techniques confirmed the presence of artwork invisible to the naked eye and preserved beneath protective mineral layers (1).

What are the implications of this study?

This study shows how newer methods for analyzing cave paintings will continue to need to be adaptive and flexible. The researchers acknowledged in their study that archaeological sites present greater variability than laboratory models, with factors such as pigment thickness, preservation levels, surface moisture, and substrate heterogeneity introducing complications (1). These environmental factors may produce noise and limit the accuracy of pigment detection, particularly when using supervised models (1).

Both techniques, therefore, are likely to see further refinement. For example, reflectance spectroscopy may be integrated with hyperspectral imaging. This integration could allow entire cave walls to be scanned in real time, providing faster and more comprehensive pigment detection (1). For magnetic susceptibility, improvements could involve pairing the sensor with inertial measurement units to speed up the currently time-intensive process of high-resolution spatial measurements (1).

By eliminating the need for invasive calcite removal, magnetic susceptibility and reflectance spectroscopy offer archaeologists a safer way to explore the hidden artistic legacy of early humans (1). Although the techniques are not yet fully operational for field deployment, their successful use in this study marks an important milestone.

This study shows that combining magnetic susceptibility and reflectance spectroscopy could open new perspectives for detecting and mapping cave paintings still buried under calcite.

References

1. Jal, M.; Durlet, C.; Monna, F.; Morales, C.; et al. Detecting pigments of cave paintings hidden beneath calcite layers: The potential of magnetic susceptibility and visible-to-infrared (400-2500 nm) reflectance spectroscopy. J. Cul. Herit. 2025, 75, 279–290. DOI: 10.1016/j.culher.2025.07.024
2. Virtual Museum of Minerals and Molecules, Calcite. Wisc.edu. Available at: https://virtual-museum.soils.wisc.edu/display/calcite/ (accessed 2025-09-08).